The friction of two materials rubbing against each other causes the “triboelectric” phenomenon known as static charge. Static buildup is common in conveying systems and printers. If the static electricity does not have a grounded path to follow it can lead to an Electro Static Discharge (ESD) which can, in turn, cause short circuiting of electronic components, induce a fire or cause difficulties in moving material paper, plastic sheeting etc. through a piece of equipment.
Many media to be printed onto, particularly polymeric sheets and the like, are insulating in nature and have a tendency to accumulate static charge. Often, to prevent such media becoming soiled, the sheets or rolls are wrapped in polyethylene or other material which is, itself insulating. Removal of such films tends to cause electrons to be transferred between wrapping and media and in consequence thereof, both become electrostatically charged. The printing process itself requires the media to be fed past the print head. Typically rollers are used and the media is stretched over and fed past a print table. Relative movement of insulating materials, particularly where there is friction, tends to cause additional static build up.
There are a large number of charge-based injection processes wherein droplets of ink are intentionally charged as a means of directive spraying. The very process of atomization, i.e. of breaking a liquid into droplets that is essential to inkjet printing shears the ink and causes charging of the droplets. It will be appreciated that inkjet printing requires injection of drops of ink through accurately machined nozzles onto the media and this causes charging of the droplets. For printing onto polymeric materials, whether plastic sheets, netting used for covering buildings with advertising, or plastic coated papers, the ink is generally oil based and highly susceptible to charge build up. Where the media is charged, the droplets get attracted or repelled by areas of charge concentration, such as defects on the micro-surface. This lowers the precision of the printing process and thus the resolution and accuracy of the end result. It is not just oil based inks of course, since each water molecule has a dipole moment, droplets of water based ink get attracted or rejected by static charges and non-controlled static build up adversely affects the resolution and quality of the printing.
It will be appreciated that the wider the print media, the more charge and the more unevenly charge can build up there-across. Thus the problems of static are exasperated by wide format printers and even more so by extra wide format printers, where the media can be from 60 cm to 3 m, 5 m or even 10 m wide.
Wide format inkjet printing is commonly used for printing posters, wall decorations, signs and the like. The medium to be printed, if flexible, is typically provided on a continuous roll and is advanced, roll to roll, past the print head. Sometimes however, particularly when printing onto a stiff material, the material to be printed is provided in sheet form and the printing technique is then known as flat-bed printing. Dual mode printers are designed for high resolution printing onto both flexible and rigid substrates. An example of such a printer, is Applicant's co-pending patent application, U.S. Ser. No. 11/693,449, incorporated herein by reference, in its entirety, which discloses a wide or super wide digital printer comprising a print head box that reciprocates from left to right across a wide or super wide printing table having a length of less than 20 cm that is supported by a fixed support, and a feed roller and a guiding roller that are moveably coupled to the fixed support, wherein the wide or super wide digital printer is alternatively configurable as (a) a roll to roll printer by lowering the feed roller and the guiding roller to a lowered configuration wherein uppermost parts of the feed roller and the guiding roller are below the upper surface of printing table, or (b) as a discontinuous sheet printer by raising the feed roller and the guiding roller to a raised configuration wherein the uppermost parts of the feed roller and the guiding roller are collinear with the upper surface of the printing table. Here again, the relative movement of the parts with respect to each other may cause charge build up.
As elegantly demonstrated in Milliken's famous oil-drop experiment, droplets can be suspended in charge fields and thus the adverse affect static charge can have on the accurate placing of inks onto print media cannot be overstated.
The current state of the art is typically to wipe a damp cloth over print media rolls once mounted onto the printer feeder and periodically to wipe such a cloth over the print head, thereby discharging static. Anti-static brushes are sometimes used as well. However, such techniques are severely limited in their effectiveness.
Korean Patent Application Number KR4002100 titled “Apparatus for Removing Static Electricity of Ink Jet Printer” describes a static electricity control apparatus for ink-jet printer having a pinch roller with shaft being rotatably supported on holder, and a ground unit for grounding the pinch roller to a metallic frame. Essentially a static electricity removing apparatus for an ink jet printer is provided to remove static electricity which is generated when a paper is conveyed by connecting a grounding member to a pinch roller. The printer includes a frame and has a paper feeding section provided at a lower portion of the frame. A feeding roller conveys paper from the paper feeding section towards a print head. A pinch roller makes contact with the feeding roller to convey the paper. The pinch roller is rotatably supported by means of a holder. A grounding member is provided to remove static electricity from the papers passing through the feeding roller and the pinch roller. One end of the grounding member is connected to a shaft and the other end of the grounding member is connected to the frame. Such a grounding system is not really suitable for wide format printers that often print onto polymer mediums.
U.S. Pat. No. 7,300,136 titled “Ink Tubing Chain Slider for Wide Format Printer” describes an ink jet printer that includes a carriage mounted on a guide. The carriage provides a transverse movement to a print-head. The carriage includes an upper carriage component adjacent to a rear support wall of the ink jet printer and a lower carriage component mounted to the upper carriage component. A slider is disposed between the upper carriage and the rear support wall. The slider includes a rigid substrate with a hole or an eyelet forming a hole. A pin secured to the carriage engages the hole and allows the slider to rotate up to 360 degrees around the pin. The slider includes a fabric with extended fibers secured to the substrate or directly to the carriage in order to resist static charge accumulations and to lower friction.
This system is designed for a wide format printer and addresses some of the requirements thereof, in particular, the accumulation of static charge due to movement of the print head carriage. It does nothing to discharge the large static charges that might accumulate from the unwrapping of the medium and from the triboelectric build up caused by the advancing system used for moving the medium past the print stage.
Japanese patent application number JP07237293A2 titled “Ink Jet Printer” describes an ink jet printer in which static electricity charged on paper is removed to protect the print head from damage.
A discharge means for discharging static electricity charged on paper is disposed on upstream and downstream sides of an ink jet head in the paper feed direction. The upstream discharge means is composed of a grounded paper feed roller and a pinch roller. The downstream discharge means is composed of a grounded transport roller for transporting paper in contact with a non-printing surface of the paper and a discharge brush. Bristle tips of the discharge brush are directed to the head out of contact with the paper. The system described is useful for discharging static of the amounts that accumulate on A4 or American foolscap paper. Unfortunately, it does not scale up to the requirements for wide format printing onto polymeric media.
Japanese patent application number JP2008044742A2 titled “Ink Jet Printer and Method of Eliminating Static Electricity from Print Medium for Ink Jet Printer” attempts to remove charges on the surface of a print medium even when the frequency of polarity change or the potential of an applied voltage for charging a conveyor belt is varied. When a voltage applied to a print medium charging roller is zero, the potential on the surface of the print medium in contact with the print medium charging roller on the opposite side of the conveyor belt is detected from the partial voltage of the voltage generated in the print medium charging roller. A print medium AC charging pattern signal (applied voltage) to the print medium charging roller is so controlled that the detected potential on the surface of the print medium on the opposite side of the conveyor belt can be suppressed. Consequently, the charges on the surface of the print medium can be effectively removed even when the frequency of the polarity change or the potential of the conveyor belt AC charging pattern signal (applied voltage) for charging the conveyor belt is varied.
The system described apparently applies a carefully controlled discharging voltage to a discharging roller. The limitations thereof, particularly for wide format printing onto polymer sheets will be apparent to persons of the art.
There is a need for more effective systems and methods of discharging static charge from print media, particularly from wide, polymeric media, and the present invention addresses this need.